Ordinary backlight units for liquid-crystal image displays and those for various guide lights generally have a built-in surface light source that emits light uniformly. The surface light source is made of a transparent tabular molding. This receives the light from a main light source, cathode ray tube (fluorescent lamp) combined with it, and emits light from its surface, and this is referred to as an optical waveguide. Specifically, the light from the main light source enters the optical waveguide through its side surface, and a part of it runs inside the optical waveguide and scatters on the light-scattering layer disposed on the back surface of the optical waveguide to give scattered light, and, as a result, the entire surface of the optical waveguide emits light uniformly.
For forming the light-scattering layer, heretofore, a dot pattern is printed on the back surface of a plate to be an optical waveguide, or the back surface thereof is machined with a conical drill. However, such methods require a high-level technique. Therefore, the recent tendency in the art is toward a method of transferring a microprism structure onto a plate to be an optical waveguide.
The material of such an optical waveguide must satisfy the requirement of high-level complete light transmittance, for which, therefore, generally used is acrylic resin (PMMA) However, acrylic resin does not have good heat resistance, high mechanical strength and good flame retardancy, and is therefore unsuitable for lighting units of, for example, displays, tail lamps and winkers for automobiles. As opposed to this, polycarbonate resin is being used for that purpose, as having good heat resistance, high mechanical strength and good flame retardancy. However, ordinary polycarbonate resin is poorly flowable, and is therefore problematic in that the transferability of a microprism structure onto a plate of the resin is not good. For improving the transferability of such polycarbonate resin, known is a method of lowering the molecular weight of the resin. However, the reduction in the molecular weight of the resin detracts from the mechanical strength thereof. Accordingly, for the material of an optical waveguide, desired is polycarbonate resin having good flowability and high mechanical strength. The present invention has been made in consideration of the situation mentioned above, and is to provide an optical waveguide made from a polycarbonate resin composition having improved flowability, and to provide a method for producing it.